Manjushree Trail Race — generated on your device
Facebook / Open Graph use ?bib=BIBNUMBER.png&cat=… as og:url (same as the Share link).
index.php answers social crawlers at that URL with meta tags; browsers still get index.html.
Saving the certificate writes {bib}_{cat}.png and {bib}.png for previews; if missing, og-certificate-image.php fills in.